On the lower right hand side of the lens, disconnect two ribbons from their sockets, one on top of the other.

(These ribbons connect to the image stabilisation cell and the iris.)

Disconnect a ribbon from its socket on the top of the lens.

(This ribbon connects to the focussing motor.)

Using a small flat screwdriver or a pin (not a knife), very gently scrape away the mastic from the focussing lens position sensor on the top of the lens, and ease the sensor out.

On reassembly, secure the sensor with a drop of nail varnish or hot melt glue applied with a cocktail stick. (A glue gun is likely to dispense far too much if applied directly.) Cyanoacrylic glue is not recommended as there have been reports that the fumes can fog lens coatings.